Goedeking, A. (Corresponding author)FZJ*

1998
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag Jülich

Report No.: Juel-3514

Abstract: Constructed wetlands are stands of helophytes with water flowing through their root zone. Phosphate rem oval in constructed wetlands is the sum effect of different retention mechanisms (sedimentation, uptake by soil and incorporation into the biom ass of microbes and vegetation). Because of the short-term net retention potential being exhausted in a few years, the ecotechnological efficiency of such systems is determined by the mechanisms of sustainable retention. The sustainable retention includes removal by plant harvest and the enrichment of inorganic and organic soil substrate. In order to simulate the phosphate removal from surface water, phosphate retention was studied in three gravel-based constructed wetlands at a given loading rate. Two systems were planted with Phragmites australis (common reed), one with Typha latifolia (wide-Ieaved cattail). Oue to the higher biomass yield, the shoots of Phragmites australis accumulated more phosphate per unit area than those of Typha latifolia. Accordingly, the rate of organic soil enrichment was increased in the Phragmites-systems. Wetlands planted with Phragmites australis are consequently more suitable for sustainable retention than those planted with Typha latifofia. The supplementary addition of dissolved organic carbon (DOC) by using straw percolate resulted in a decreased phosphate retention. The dosage of straw percolate reduced phosphate storage in the soil by preventing phosphate sorption to poorly crystallized iron oxides and hydroxides. Concerning the plants, the addition certainly led to an increased biomass yield, but simultaneously reduced the specific phosphorus content. Consequently, this resulted in a decreased storage of phosphate until the end of the growing season. If such systems are used for surface water polishing, the addition of dissolved organic carbon should, therefore, be avoided. If a constructed wetland would be managed in the most effective operating mode (plants: Phragmites australis; regular plant harvest in winter, no accessory OOC inputs) with surface water containing 200 Ilg Ptot/I (Ioading rate: 2,8 g P/(m$^{2}\cdot$a)) in a region with temporate climate, a phosphate removal of 94 % is to be expected for at least 120 years. In this case approximately 11 m$^{3}$ water can be polished annually by one square meter of the system, and the phosphate content is lowered to a concentration that corresponds to a reduction from an eu-/hypertrophie to a moderate eu-/oligotrophic level.

1. Institut für Chemie und Dynamik der Geosphäre (ICG)